When spent liquors from the pulp industry, in particular so-called black liquor, which is present in concentrated form, are combusted in a recovery boiler, the water evaporates while the organic material in the liquor, mainly lignin residues, is combusted and inorganic chemicals, in smelt form, run down along the walls and collect in the bottom of the boiler. These chemicals are recovered and used again in the pulp cooking.
However, during the combustion, it is impossible to prevent dust from accompanying the flue gases and becoming stuck to the tubes in the heat exchanger which the flue gases have to flow through. This dust is brushed off and collected as ash. It consists princi pally of sodium sulphate to an extent of about 90%, chlorides and carbonates to an extent of 7-8% and potassium to an extent of 3-4%. While the sodium sulphate can be reused, the potassium, which is principally derived from the wood, and the chlorides and carbonates, have to be removed.
Several processes for purifying this ash have been disclosed. Most of them are based on leaching the ash in a container, in association with which chlorides, potassium salts and the like go in solution. This leaching can take place both at an elevated temperature and at a lower temperature, for example less than 20.degree. C. Processes of this nature are described, for example, in WO96/12847, the published Japanese Patent Application No. 311462/1994 and the U.S. Pat. No. 5,352,332.
If a leaching is to provide the desired result, such that the contaminants are leached out from the substance which it is desired to retain in a solid state, the contaminants have to be soluble in the leaching medium, in this case water, while the solid substance which it is desired to retain has to be of lower solubility. However, sodium sulphate is also soluble in water and there will therefore be losses during the leaching. However, the solubility of sodium sulphate decreases as the temperature decreases, in contrast to the solubility of potassium salts, which is virtually independent of the temperature. The chlorides are also readily soluble at both high and low temperatures. The problem with the above-disclosed processes has been that it has not been possible to achieve the desired selectivity between potassium and sodium salts, i.e. such that the sodium sulphate can be recovered from the leaching process and supplied to black liquor for combustion while the potassium salts remain in solution.
Another problem with the known leaching methods is that, if these methods are carried out in lukewarm or cold water, i.e. at less than approx. 32.degree. C., when the solubility of the sodium sulphate is low, the sodium sulphate will become surrounded by water molecules and form sodium sulphate heptahydrate or, more usually, sodium sulphate decahydrate. If the sodium sulphate is returned to the black liquor in this form, the concentrated black liquor will then be diluted with this water, something which naturally constitutes an undesirable problem.
There has therefore long been a great need to be able to utilize the ash from the recovery boiler by being able to return practically all the sodium sulphate, virtually free of potassium salts and chlorides, to the black liquor, without dilution water being supplied to the black liquor, and a process has been developed, in accordance with the invention, for purifying ash from a recovery boiler, which ash consists principally of sodium sulphate, by leaching or evaporation-crystallizing contaminants consisting mainly of chlorides and potassium salts in water, which process is characterized in that the leaching or the evaporation-crystallization takes place in an aqueous solution at a temperature of greater than approximately 32.degree. C., preferably in the vicinity of the boiling point of the aqueous solution, with leached or evaporationcrystallized ash being returned to the black liquor which is to be combusted or returned directly into the recovery boiler, with the leaching water or the evaporation-crystallization liquid being cooled to less than approximately 32.degree. C., preferably to approximately 10-15.degree. C., with the sodium sulphate con-taining water of crystallization, such as sodium sulphate decahydrate or sodium sulphate heptahydrate, being crystallized out and then separated in order to be reintroduced into the black liquor, directly into the recovery boiler or into the leaching solution or evaporation-crystallization solution.
According to the process, it is expedient for the ratio between ash and water in the leaching apparatus to be approximately 1:1.
According to the process, the leached or evaporation-crystallized ash can expediently be subjected to a washing in water or an organic solvent which dispels the water.
According to the invention, it is also expedient in many cases for water to be added to the leaching or evaporation-crystallization water before or during the cooling-down in order to ensure that sufficient water is present for the crystallization.